As long as we are on the subject, in macOS 11.5 System Preferences > Startup Disk sees and external boot drive created by the Big Sur installer but does not see a bootable external clone. It is early days in testing but at this point I have no idea what the problem is and whether it is ASR vs Installer, or a startup Disk issue. I do know Startup Disk is inconsistent in what drives it sees. Not only is it inconsistent in what drive options it sees attempting to use it to change boot drive selections finally forced me to use the Fallback Recovery Drive to get correctly rebooted. Either (I ham handed something or this is still a work in process but I was starting to sweat before I was able to boot back into the internal NVME).
On a similar subject I have been successful in creating a bootable USB SSD by connecting through the Thunderbolt 3 port in the Thunderbolt monitor using Thunderbolt 4 cables on both sides. So it is definitely possible, even easy, but connection and cable sensitive.
